1. Field of the Invention
The invention relates to a diffusor for an axial-flow turbo-machine, in which the kink angles of the diffusor inlet both at the hub and at the cylinder of the turbo-machine are determined solely for the purpose of equalizing the total pressure profile over the channel height at the outlet of the last blade row, means are provided for canceling the rotation of the rotational flow, in the form of flow ribs within the deceleration zone of the diffusor, and at least one flow-guiding guide plate is provided for subdividing the diffusor.
2. Discussion of the Background
Diffusors of this type for turbo-machines are known from EP-B-265 633. In order to meet the requirement for the best possible pressure recovery and nonrotational diffusor flow-off under a full load and a part load, a straightening cascade extending over the entire height of the channel through which the flow passes is provided within the diffusor. These means for canceling rotation are cylindrical flow ribs arranged uniformly over the circumference and having thick straight profiles which are designed in light of the knowledge of turbo-machine building and which are to be as insensitive as possible to an oblique onflow. The flow-facing front edge of these ribs is located relatively far behind the outlet edge of the last moving blades in order to prevent any excitation of the last blade row due to the pressure field of the ribs. This distance is calculated so that the front edge of the ribs is located in a plane in which a diffusor surface ratio of preferably three prevails. This first diffusion zone between the blading and the flow ribs is therefore to remain undisturbed as a result of complete rotational symmetry. That no interference effects are to be expected between the ribs and blading is attributable to the fact that the ribs take effect only in a plane in which a relatively low velocity level already prevails.
Since, in the case of conventional highly loaded bladings of turbines, their opening angle far exceeds that of a good diffusor, in order to assist the flow the known diffusor is subdivided in the radial direction into a plurality of part diffusors by means of flow-guiding guide rings. These guide rings extend from a plane directly at the outlet of the blading to a plane in which a diffusion ratio of three is obtained, that is to say over the entire first diffusion zone. For reasons of vibration, these guide rings are preferably designed to be in one part. This leads to a design without a parting plane, which is disadvantageous for assembly reasons. Furthermore, in the case of large machines, the guide rings result in large diameters, so that transport problems can arise.
A second diffusion zone extends from the front edge of the thick flow ribs as far as the largest profile thickness of the ribs. In this second zone, the cancellation of rotation of the flow is for the most part to be carried out largely without any deceleration. In a third downstream diffusion zone in the form of a straight diffusor, a further deceleration of the flow, virtually nonrotational at that moment, takes place.
All these measures are intended to achieve not only a maximum pressure recovery, particularly under part load, but also a reduction in the overall length of the plant.
In conventional gas turbines, the flow reaches the diffusor under no load at a velocity ratio c.sub.t /c.sub.n of approximately 1.2, c.sub.t signifying the tangential velocity and c.sub.n the axial velocity of the medium. This oblique onflow leads to a decrease in the pressure recovery C.sub.p.
In other machine types such as, for example, steam turbines, it is possible that the volume flow is reduced to 40% and therefore c.sub.t /c.sub.n ratios up to 3 occur. In such machine types, a fixed diffusor geometry is inappropriate, since the pressure recovery could even become negative. This applies even when the ratio of spacing to chord of the flow ribs amounts to 0.5. Flow ribs with spacing/chord ratios of approximately 1, which would occur under full load, that is to say c.sub.t /c.sub.n =approx. 0, specifically a somewhat higher pressure recovery, cannot be used at all in such machines.
The pronounced decrease in the pressure recovery is attributable to the fact that, at the extreme ratios mentioned, a strong vortex forms between the outlet moving blades and flow ribs. The vortex is limited by the flow ribs on which the tangential component of the velocity is dissipated. If solid particles, for example water droplets in steam turbines, are carried along on the backflow which is established, an acute risk of foot erosion on the blades of the last moving-blade row can arise.